This invention relates generally to a method of and apparatus for detecting a characteristic in a stream of particulate material and more particularly to the detection of radioactive material in a stream of material. The invention lends itself to the measurement of grade of radioactive material for example in radioactive ore sorting, in particle or bulk sorting applications and bulk monitoring applications.
In known methods of detecting gamma radiation from radioactive material either single or multiple scintillation detectors have been used, these being mounted under a conveyor belt carrying the material or under the trajectory of material in free flight. These detectors provide a measure of the grade of radioactive mineral content of the material and enable one to sort the material into, accept and reject fractions or to monitor continuously the grade of a bulk stream of the material.
It has also been proposed to pass the material through a ring detector, or among a number of detectors arranged around the material path, as is done for example in U.S. Pat. No. 3,052,353. Other detector arrangements are disclosed in U.S. Pat. Nos. 4,317,521, 3,237,765, 3,075,641, and 2,717,693, and British patent specifications Nos. 1479972, 1218844, 1030203, 960408, 939537, 855678 and 658572. In general these methods have the inherent shortcoming that the detection efficiency varies considerably with the transverse position of a radioactive ore particle in relation to the scintillation detector or detectors and, in bulk sorting applications, the detection efficiency is also dependent upon the vertical position and the distance of a radicactive ore particle in relation to the detectors. This shortcoming manifests itself as an error in the calculation of the grade of a particle or the average grade of a bulk stream and can cause the grade estimation to be in error by a factor of up to five times.
Known particle sorters have used a technique of compensating the apparent grade of the particle by a factor which is dependent on the transverse or lateral position of the particle, relative to the detectors, but this technique is not always accurate and can result in a particle being assigned a grade materially different from its actual grade.